Method for fabrication of copper-indium gallium oxide and chalcogenide thin films

ABSTRACT

A composition of matter and method of forming copper indium gallium sulfide (CIGS), copper indium gallium selenide (CIGSe), or copper indium gallium telluride thin film via conversion of layer-by-layer (LbL) assembled Cu—In—Ga oxide (CIGO) nanoparticles and polyelectrolytes. CIGO nanoparticles are created via a flame-spray pyrolysis method using metal nitrate precursors, subsequently coated with polyallylamine (PAH), and dispersed in aqueous solution. Multilayer films are assembled by alternately dipping a substrate into a solution of either polydopamine (PDA) or polystyrenesulfonate (PSS) and then in the CIGO-PAH dispersion to fabricate films as thick as 1-2 microns. After LbL deposition, films are oxidized to remove polymer and sulfurized, selenized, or tellurinized to convert CIGO to CIGS, CIGSe, or copper indium gallium telluride.

PRIORITY CLAIM

The present application is a non-provisional application claiming thebenefit of U.S. Provisional Application No. 61/977,206, filed on Apr. 9,2014 by Walter J. Dressick et al., entitled “METHOD FOR FABRICATION OFCOPPER-INDIUM GALLIUM OXIDE AND CHALCOGENIDE THIN FILMS,” the entirecontents of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to depositing copper-indium-galliumchalcogenide films using aqueous solutions of precursors.

1. Description of the Prior Art

Quaternary chalcogenide semiconductors of structure CuA_(x)B_(1-x)Z₂(where A, B=In, Ga or Zn, Sn; Z=S, Se, or Te; 0≦x≦1) are among theleading materials candidates under study as absorber layers forconversion of visible and near infrared solar radiation into electricityin photovoltaic devices. These materials offer several importantadvantages, including composition tunable band gaps for light absorptionmatched to the solar spectrum, a large knowledge base of theirfundamental properties accumulated over decades of research, andphotochemical, chemical, and thermal stability, among others. However,achieving sufficient power/energy conversion efficiencies (i.e., >20%)using appropriate materials and systems that can be prepared at low costremain fundamental barriers to photovoltaic commercialization.

With regard to the latter, a key issue in lowering costs is the abilityto prepare high quality materials and films in large quantities usingprocesses amenable for high throughput manufacturing. Although vacuumtechniques such as sputtering and co-evaporation offer exquisite controlover composition and deposition of semiconductor absorbers, they remainlargely more costly batch processing techniques. Consequently,significant efforts are being expended to develop alternative non-vacuumsynthetic and film deposition routes based on liquid phase processescompatible with high throughput manufacturing. These include suchdiverse well-developed technologies as electrodeposition,sol-gel/chalcogenization, reactive solution deposition, interfacialself-assembly, and spincoating, dipcoating, doctor-blading, or inkprinting, alone or in combination with subsequent thermal annealingtreatments.

One increasingly popular liquid phase dipcoating technology compatiblewith high throughput manufacturing is layer-by-layer (LbL) deposition.LbL films are formed via alternate exposure of a substrate to separateaqueous solutions or dispersions containing oppositely multi-chargedspecies. A surface charge reversal occurs during substrate treatmentwith each species, allowing controlled conformal electrostaticdeposition of the oppositely-charged material during the next step. Forexample, polymer multilayer films are readily prepared using solutionsof cationic and anionic polyelectrolytes, with film thickness,structure, and morphology controlled by pH, added salt type, ionicstrength, polyelectrolyte molecular weight, and/or temperature duringthe deposition process. Replacement of one or both polyelectrolytesolutions by appropriately charged nanoparticle dispersions permitsfabrication of composite materials.

With regards to solar energy applications, the LbL technique has beenincreasingly exploited for device fabrication, albeit on smaller scalesoften constrained by the availability of large amounts of the componentnanoparticles. For example, Lee et al. have deposited SnO₂nanoparticle/polyallylamine (PAH) multilayers that were sintered toprepare SnO₂ films useful for cascadal energy band gap matching in dyesensitized solar cells (DSSCs). (Kim, et al., “Effect of Layer-by-LayerAssembled SnO₂ Interfacial Layers in Photovoltaic Properties ofDye-Sensitized Solar Cells,” Langmuir, 28, 10620-10626 (2012)).Furthermore, Ruhlmann et al. have recently fabricated DSSCs via an LbLapproach using polyoxometalate and porphyrin dye components. (Ahmed etal., “A molecular photovoltaic system based on Dawson typepolyoxometalate and porphyrin formed by layer-by-layer self assembly,”Journal of the Chemical Society-Chemical Communications, 49, 496-498(2013)). Zotti et al. have prepared photovoltaic cells from LbLmultilayers comprising PbSe nanocrystals and polyvinylpyridine,evaluating semiconductor stability and properties usingphotoelectrochemical and photoconductivity techniques. (Vercelli et al.,“Self-Assembled Structures of Semiconductor Nanocrystals and Polymersfor Photovoltaics. PbSe Nanocrystal-Polymer LBL Multilayers. Optical,Electrochemical, Photoelectrochemical, and Photoconductive Properties,”Chemistry of Materials, 22, 2001-2009 (2010)). In similar fashion, Noziket al. have described LbL deposition of Schottky solar cell devicesprepared from PbSe nanocrystals and ethanedithiol crosslinkers, with anunsintered device exhibiting 2.1% efficiency. (Luther et al., “Schottkysolar cells based on colloidal nanocrystal films,” Nano Letters, 8,3488-3492 (2008)). More recently, Srestha et al. have reported aphotovoltaic device incorporating a multilayer absorber layer preparedvia LbL deposition of polyethylenimine (PEI) and polystyrenesulfonate(PSS)-coated copper-indium-gallium selenide (CIGSe) nanoparticles,demonstrating ˜3.5% efficiency for the non-optimized device. (Hemati etal., “Layer-by-Layer Nanoassembly of Copper Indium Gallium SeleniumNanoparticle Films for Solar Cell Applications,” Journal ofNanomaterials 2012: Article No. 512409 (2012)).

BRIEF SUMMARY OF THE INVENTION

The aforementioned problems are overcome in the present invention whichprovides a layer-by-layer approach for deposition ofcopper-indium-gallium chalcogenide (chalcogenide=sulfide, selenide, andtelluride) films using aqueous solutions of precursors. The process usescopper-indium-gallium oxide (CIGO) nanoparticles, which can be producedin gram scales via flame spray pyrolysis of alcohol solutions containingCu(II), In(III), and Ga(III) salts, as a starting material. Polyaminepolycations, such as polyallylamine (PAH), are covalently bound viasonication to the CIGO particles to provide a stable aqueous dispersionof polyamine-coated CIGO nanoparticles, such as CIGO-PAH nanoparticles,which are then electrostatically adsorbed to substrates coated with apolyanion film, such as polystyrenesulfonate (PSS) or polydopamine(PDA). Alternate treatments of the substrate with the CIGO-polyaminedispersion and the polyanion solution permit fabrication of thincomposite films with controllable thicknesses. Subsequent thermaloxidation removes the organic polyelectrolyte components, leaving a pureCIGO film that is subsequently converted to CIGS films by H₂S vaportreatment.

The present invention has many advantages. It provides a simple, cheap,automatable layer-by-layer aqueous process to fabricate CIGS absorberlayers from abundant CIGO nanoparticle precursors and polyelectrolytes.Other CIGS deposition methods rely on vacuum processing (high capitalexpense) or on nanoparticle solutions or metallic inks that usehazardous solvents and are difficult to produce. This technique relieson nanoparticles that are readily producible by flame-spray pyrolysis inkilogram-scale quantities. The combination of flame spray pyrolysis andautomatable non-vacuum layer-by-layer deposition provides a simple,low-cost, scalable method compatible with high throughput manufacturingto fabricate CIGS absorber layers from readily available precursors.

These and other features and advantages of the invention, as well as theinvention itself, will become better understood by reference to thefollowing detailed description, appended claims, and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows CIGO nanoparticle characterization. FIG. 1A shows TEM offlame spray pyrolysis (FSP) as-prepared CIGO nanoparticles. FIG. 1Bshows XRD patterns for the CIGO sample prepared by FSP in part A andCIGS species obtained after its sulfurization.

FIG. 2 shows CIGO-PAH dispersion characterization. FIG. 2A showsparticle size distribution and concentration for the fresh 1CIGO-PAH·mL⁻¹ (aq) dispersion determined using the NanoSight LM10-HSBFnanoparticle tracking system. FIG. 2B shows UV-visible absorbancespectrum of a freshly prepared 0.33 mg CIGO-PAH.mL⁻¹ aqueous dispersionin a b=0.10 cm pathlength cuvette vs. a water baseline. The peak at 285nm corresponds to incompletely oxidized species within the particles.

FIG. 3 shows aqueous CIGO dispersion stability. Particle size dispersionhistograms from dynamic light scattering (DLS) measurements illustratingthe time dependent aggregation of the quiescent 1 mg CIGO-PAH.mL⁻¹ (aq)dispersion are shown in FIG. 3A aged for 3 days; in FIG. 3B aged for 3days, after mixing; in FIG. 3C aged for 6 days, 10 min after mixing; inFIG. 3D aged for 6 days, 40 min after mixing; in FIG. 3E aged for 7days, 10 min after mixing; and in FIG. 3F aged for 13 days, 10 min aftermixing. Particles larger than 300 nm were set as the aggregationthreshold.

FIG. 4 shows aqueous CIGO dispersion stability in pH 8.25 buffer.Particle size histograms from DLS illustrating the time dependentaggregation of the 1 mg CIGO-PAH.mL⁻¹ 20 mM Tris pH 8.25 (aq) dispersionare shown in FIG. 4A at preparation (t=0 min), in FIG. 4B at t=30 minafter preparation, in FIG. 4C at t=50 min after preparation, and in FIG.4D at t=420 min after preparation. Insets in FIGS. 4C and 4D areexpansions of the particle distributions for particles larger than ˜400nm diameter.

FIG. 5 shows a film fabrication scheme using CIGO-PAH colloids and PSSor PDA polyelectrolytes. The process sequence (not to scale) includes:(1) Treatment of substrate with PSS (aq) or pH 8.25 dopamine (aq)solution (for in situ PDA generation) followed by CIGO-PAH (aq)dispersion or CIGO-PAH/Tris pH 8.25 (aq) dispersion to deposit firstbilayer of PSS/CIGO-PAH or PDA/CIGO-PAH, respectively. Repetition oftreatment cycle deposits additional bilayers for multilayer filmfabrication; (2) Air oxidation for 5 h at 550° C. to remove organiccomponents from CIGO particles; and (3) Sulfurization for 3 h at 550° C.in H₂S to convert CIGO particles to CIGS film.

FIG. 6 shows characterization of PSS/CIGO-PAH multilayers prepared byhand dipcoating EDA-coated quartz slides (Q-EDA, whereEDA=N-(2-aminoethyl)-3-aminopropyltrimethoxysilane). FIG. 6A showsabsorbance spectra in descending order at 285 nm of PSS/CIGO-PAHmultilayer films of structure Q-EDA/(PSS/PAH)₃/(PSS/CIGO-PAH)_(x) withx=18 (top line), x=18 after annealing in air 5 h at 550° C. (dashedline), x=12 (second to bottom line), and x=6 (bottom line). The measuredabsorbance represents films having the structures shown that are presenton both sides of the quartz slide. FIG. 6B shows absorbance vs. numberof bilayers, x, for Q-EDA/(PSS/PAH)₃/(PSS/CIGOPAH)_(x) multilayers. Thesquares (225 nm) and circles (285 nm) indicate a film initiated using 2day aged CIGO-PAH; the diamonds (225 nm) and triangles (285 nm) indicatea film initiated using fresh CIGO-PAH.

FIG. 7 shows characterization of PSS/CIGO-PAH multilayers prepared byrobot dipcoating. FIG. 7A shows absorbance vs. number of bilayers, x,for Q-EDA/(PSS/CIGO-PAH)_(x) multilayers prepared via automateddipcoating using the robot. FIG. 7B shows absorbance spectra inascending order at 400 nm of robot dipcoated PSS/CIGO-PAH multilayerfilms of structure Q-EDA/(PSS/CIGO-PAH)₈₀ after annealing in air 5 h at550° C. (bottom line), as deposited (middle line), and after H₂Ssulfurization 5 h at 550° C. (top line). The measured absorbance derivesfrom films having the structures shown that are present on both sides ofthe quartz slide.

FIG. 8 shows deposition of polydopamine thin films. FIG. 8A showsabsorbance spectrum of polydopamine (PDA) film on a Q-EDA slide preparedby 90 min treatment in freshly made 1 mg dopamine·mL⁻¹10 mM Tris pH 8.25(aq) solution. Spectrum shown after subtraction of an untreated Q-EDAslide baseline. The measured absorbance represents PDA films that arepresent on both sides of the quartz slide. FIG. 8B shows time dependentabsorbance at 285 nm for Q-EDA slide treated with 1 mg dopamine.mL⁻¹ 10mM Tris pH 8.25 (aq) buffer solution.

FIG. 9 shows characterization of PDA/CIGO-PAH multilayers prepared byhand dipcoating. FIG. 9A shows absorbance vs. number of bilayers, x, forQ-EDA/(PDA/CIGO-PAH)_(x) multilayers deposited by hand dipcoating. FIG.9B shows absorbance spectra in ascending order at 600 nm of handdipcoated PDA/CIGO-PAH multilayer films of structureQ-EDA/(PDA/CIGO-PAH)₂₀ after annealing in air 5 h at 550° C. (bottomline) with thickness 953±162 nm, as deposited (middle line) withthickness 1163±189 nm, and after sulfurization 5 h at 550° C. (top line)with thickness 1079±168 nm. Film thicknesses were measured byprofilometry. The measured absorbance derives from films having thestructures shown that are present on both sides of the quartz slide.

FIG. 10 shows SEM images of Si-EDA/(PDA/CIGO-PAH)₂₆/PDA films. FIG. 10Ais a top view for as-deposited film. FIG. 10B is a side view foras-deposited film. FIG. 10C is a top view for as-deposited film after 5h air oxidation at 550° C. FIG. 10D is a side view for as-deposited filmafter 5 h air oxidation at 550° C. FIG. 10E is a top-view for oxidizedfilm after sulfurization 3 h in H₂S at 550° C. FIGS. 10F and 10G areside views for oxidized film after sulfurization 3 h in H₂S at 550° C.

FIG. 11 shows XRD of the CIGS film shown in FIGS. 9E-G.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a unique approach usingcopper-indium-gallium oxide (CIGO) nanoparticles, which are readilyproduced in large quantities, in combination with automatable LbLdeposition methods for the low cost manufacture of copper-indium-galliumoxide (CIGO) and copper-indium-gallium chalcogenide (such ascopper-indium-gallium sulfide (CIGS), copper-indium-gallium selenide(CIGSe), and copper-indium-gallium telluride) films. This methodinvolves gram scale preparations of CIGO nanoparticles using a flamespray pyrolysis (FSP) technique, their subsequent surface modificationvia binding of polyallylamine (PAH), and the formation of stable aqueousdispersions prepared from the resulting CIGO-PAH colloids. Compositemultilayer films comprising the CIGO-PAH colloids, together withpolystyrenesulfonate (PSS) or polydopamine (PDA), are further preparedvia an aqueous LbL approach and characterized. Subsequent oxidation toremove the organic components and sulfurization to form CIGS films foruse as light absorber layers in photovoltaic devices are also described.

Described herein are procedures for the flame spray pyrolysis (FSP)preparation of gram scale quantities of CIGO particles having diametersof ˜10-75 nm and tunable compositions from simple solution precursors.Subsequent binding of polyallylamine (PAH) to the particle surface viasonication is accompanied by limited extraction of Cu and Ga from theparticle surface and particle aggregation, with the resulting ˜125 nmdiameter CIGO-PAH species forming aqueous colloidal dispersions whosestabilities depend on dispersion age and pH. Dispersions aresufficiently stable for reproducible fabrication of compositePSS/CIGO-PAH multilayer films with PSS via a layer-by-layer handdipcoating approach. Automation of the process using a robot dipcoaterhas been demonstrated, providing films containing as many as 80PSS/CIGO-PAH absorbing >90% of all light of λ≦1100 nm after oxidationand sulfurization. The analogous PDA/CIGO-PAH multilayers are alsoreadily prepared from the CIGO-PAH dispersions via hand-dipcoating usingPDA, generated in situ by dopamine polymerization, rather than PSS.

Materials

All chemicals were used as received from the indicated sources unlessotherwise noted. Deionized water (18.2 MΩ·cm) for all experiments wasprepared by passing water through an Elix® 5 Milli-Q Plus Ultra-PureWater System (Millipore Corp.). Nitrogen gas was obtained from in-houseliquid N₂ boil-off and passed through a cellulose filter prior to use.Methane and oxygen gas for flame spray pyrolysis (FSP) experiments andhydrogen sulfide gas for the sulfurization experiments were from AirgasInc. Acetone (No. 179124), methanol (No. 179337), hydrochloric acid (No.320331), sulfuric acid (No. 320501), glacial acetic acid (No. 320099),sodium hydroxide ≧99.99%, Electronics Grade, No. 306576), sodium2-mercaptoethanesulfonate (MES, No. M1511), cysteamine hydrochloride(No. M6500), 3-mercaptopropionic acid (No. M5801), sodium chloride (No.59888), dopamine hydrochloride (No. H8502),tris(hydroxymethyl)aminomethane (Tris, No. T87602), polyethylenimine(PEI, No. 181978, 750,000 g·mole⁻¹), polyallylamine hydrochloride (PAH,No. 283223, 70,000 g·mole⁻¹; No. 283215, 15,000 g·mole⁻¹; No. 283215,8,000-11,000 g·mole⁻¹), and polystyrenesulfonate (No. 243015, 70,000g·mole⁻¹) were all ACS Reagent Grade except where otherwise noted fromAldrich Chemical Co. Polyallylamine hydrochloride (PAH, No. 43092,120,000-200,000 g·mole⁻¹) was from Alfa-Aesar. Anhydrous ethanol was 200proof from the Warner-Graham Company.N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (EDA, >95%, No.SIA0591.0) from Gelest Inc. was purified by vacuum distillation(140-142° C.; 14-15 mm Hg). High purity (≧99.999%) copper (II) nitratehydrate (No. 229636), gallium (III) nitrate hydrate (No. 229644), andindium (III) nitrate hydrate (No. 254215) from Aldrich Chemical Co. wereused for the preparation of copper-indium-gallium oxides (CIGO) in theFSP experiments.

Silicon wafers (100 mm diameter, 500-550 μm thickness, [100]orientation, p-type (B doping), resistivity 6-9Ω, No. PD7403) fromWacker Siltronic Corp. were cut into 25 mm×50 mm pieces for use in theexperiments. Polished quartz slides (No. 20200, 25.4 mm×50.8 mm×1 mm)were from Quartz Scientific Inc. Silicon wafers and quartz slides werecleaned per the literature method (Dressick et al., “Proximity x-raylithography of siloxane and polymer films containing benzyl chloridefunctional groups,” Journal of Vacuum Science & Technology A-VacuumSurfaces and Films, 17, 1432-1440 (1999)) via successive 30 minimmersions in 1:1 v/v HCl/CH₃OH and concentrated H₂SO₄, with copiouswater rinsing after each treatment. An EDA self-assembled monolayer(SAM) was chemisorbed onto the freshly cleaned Si wafer and quartzsubstrate surfaces by treatment for 25 min in 1% (v/v) EDA aqueoussolution containing 1 mM acetic acid, followed by a triple water rinse,drying in the filtered N₂ gas stream, and a 6 min bake at 110° C. tocomplete the chemisorption process. (Dressick et al., “Characterizationof a colloidal Pd(II)-based catalyst dispersion for electroless metaldeposition,” Colloids and Surfaces A-Physicochemical and EngineeringAspects, 108, 101-111 (1996)).

Molybdenum foil (≧99.9%, No. 357200-25.6G, Aldrich Chemical Co., 50mm×50 mm×1 mm) substrates were cut into 25 mm×50 mm pieces for use inexperiments and then degreased by successive 5 min rinses in acetone,methanol, and water. The substrates were next polished to remove surfaceoxides and stains using 15 μm alumina grit followed by 3 μm diamondgrit. The polished Mo substrates were rinsed successively with water,isopropanol, and acetone, followed by sonication in water for 15 min at80 W power using a Branson Model 2510 Ultrasonic Cleaner/Water Bath todislodge any polishing grit adhering to the surface before drying in afiltered N₂ gas stream. The Mo substrates exhibited a near minor finishwith very faint visible scratches after processing. EDA-coated Si wafersand quartz substrates and polished Mo substrates were each stored inFluoroware™ containers until need for experiments. The stored, polishedMo substrates were soaked 1 h in 1/1 v/v HCl/methanol and rinsed withwater immediately before deposition of multilayer films. The EDA-coatedquartz slides and Si wafers were removed from storage and used directlyfor multilayer depositions.

Instrumentation

Sonication reactions of polyelectrolyte and CIGO particles were carriedout using a Sonicators & Materials Inc. Vibra-Cell Sonicator equippedwith a titanium horn. All pH measurements were made using a CorningPinnacle Model 530 pH meter. A Sorvall Model RCSB Plus RefrigeratedUltracentrifuge equipped with a Sorvall Model SS-34 rotor was used forall centrifugations. CIGO-PAH pellets formed during centrifugation werere-dispersed in water using a Scientific Industries Inc. Model Vortex-2Genie vortexer. Freeze drying experiments were performed using a VirTisInc. benchtop K freeze dryer. Automated deposition of some films wascarried out using a StratoSequence VI Robot Dipcoater from nanoStrataInc.

UV-visible absorbance spectra were acquired using a double beam VarianCary 5000 spectrophotometer. Film spectra were referenced to anEDA-coated quartz slide baseline. Absorbance spectra of varioussolutions and the CIGO-PAH dispersion were acquired using the sameinstrument with 0.10 cm pathlength quartz cells referenced to a waterblank baseline. Thermogravimetric measurements of the CIGO and CIGO-PAHparticles were made using a TA Instruments Inc. Hi res TGA2950Thermogravimetric Analyzer. CIGO-PAH dispersions were characterizedusing a NanoSight LM10-HSBF nanoparticle characterization system alongwith corresponding software Nanosight NTA2.2 for particle concentrationdeterminations and a Brookhaven Instruments ZetaPALS dynamic lightscattering (DLS) system equipped with 1 cm path-length cuvettes forparticle size measurements.

X-Ray Photoelectron Spectroscopy (XPS) measurements was acquired using aThermo Scientific K-alpha XPS system equipped with Al k-α source at 1486eV. A sample spot size of 400 μm was used. Scanning electron microscopy(SEM) was performed using a Carl Zeiss SMT Supra 55 electron microscopewith a Princeton Gamma Tech EDS detector for compositional analysis offilms. A Scintag XDS 2000 diffractometer using Cu-kα radiation andRigaku SmartLab x-ray diffractometer using Cu-Kα radiation were used tocollect x-ray diffraction spectra for phase identification of theas-prepared CIGO particles and CIGS films, respectively. A KLA-TencorAlphaStep D-120 profilometer was used to measure the thickness of thevarious as-prepared, oxidized, and sulfurized films. A custom Kurt J.Lesker Octos cluster tool was used for film sputtering and electron beamevaporation in the preparation of the photovoltaic test device. A JEOLJEM-2200FS field emission electron microscope was used to image theas-prepared CIGO particles dropcast as an ethanol suspension onto a SPI200 mesh holey carbon coated Cu TEM grid. CIGO Particle Preparation

CIGO nanoparticles of composition CuIn_(x)Ga_(1-x)O₂ (0≦x≦1) wereprepared using ethanol stock solutions containing appropriate ratios ofthe copper, indium, and gallium nitrate precursors. For the x≅0.7composition (best matched to the solar spectrum upon conversion to CIGS)of particular interest here, a stock solution was prepared by dissolving20.0 g (˜0.1066 moles, anhydrous) copper (II) nitrate hydrate, 23.28 g(˜0.0774 moles, anhydrous) indium (III) nitrate hydrate, and 9.16 g(˜0.0358 moles, anhydrous) gallium (III) nitrate hydrate in 200 mLethanol. The stock solution was fed through a homemade flame sprayapparatus nozzle at flow rates of 5 mL·min⁻¹ with the aid of an O₂dispersion gas/oxidant under a flow rate of 5 L·min⁻¹. Small pilotflames ignited from flowing 1.5 L·min⁻¹ CH₄ and 3 L·min⁻¹ O₂ and forminga ring pattern were used as the flame ignition source and as asupporting flame for the larger central flame. The pilot flame ringletsurrounded a central capillary tube that sprayed the precursor solutionmixed with oxygen dispersant gas to form precursor droplets thatunderwent combustion in the large central flame. The CIGO powders wereeither deposited directly on Mo-coated sodalime glass substrates heatedby the flame to 400° C. or collected on glass fiber filter paper mountedin a water cooled stainless steel collection chimney. For the case ofdeposition on the filter media the powders were removed by scraping witha Teflon spatula.

Polyelectrolyte Binding to CIGO

The following general procedure, illustrated for PAH (molecular weight8,000-11,000 g·mole⁻¹), was used for reaction of all aminepolyelectrolytes with the CIGO particles. The procedure for reaction ofPSS was identical, with the exception that the PSS solution pH was ˜6.8rather than the ˜8.2-8.3 used for polyamines.

A 5 mg PAH.mL⁻¹ stock solution was prepared by dissolving 1.250 g PAH in˜230 mL 1.00 M NaCl (aq) solution in a beaker with stirring. A freshlyprepared ˜6 M NaOH (aq) solution was added dropwise with stirring untilthe pH=8.3±0.1. The solution was transferred to a 250 mL volumetricflask, diluted to the mark with 1.00 M NaCl (aq) solution, and stored ina sealed glass bottle under N₂ atmosphere until needed for experiments.The 5 mg PAH.mL⁻¹ 1.00 M NaCl (aq) solution had a pH=8.2±0.1. Similarstock polyelectrolyte solutions were prepared using PEI and PSS (at pH˜6.8) in 1.00 M NaCl (aq) as necessary.

A 125 mg sample of CIGO powder and ˜80 mL of stock 5 mg PAH.mL⁻¹ 1.00 MNaCl (pH ˜8.2) solution were added to a 150 mL high walled Pyrex™ beaker(˜80 mm height×˜55 mm diameter) resting on a lab jack and securelyclamped in place in a well-ventilated fume hood. The height of liquid inthe beaker was ˜35 mm. The sonicator horn was immersed into the mixturesuch that the bottom of the horn was fixed ˜18-20 mm above the bottom ofthe beaker, raising the liquid height to ˜40 mm. The horn assembly wassecurely clamped in place and the beaker was loosely wrapped with an Alfoil cone extending ˜10 cm above the top of the beaker as a splashguard. The sonicator power was set to 480 W and the mixture wassonicated for 30 min. Following sonication, the horn and Al foil conewere quickly removed and the temperature of the gray-black dispersionwas measured by thermometer. A temperature of ˜50-55° C. was routinelymeasured, consistent with heating during sonication that reduced thedispersion volume to ˜60-65 mL.

The dispersion was allowed to cool to room temperature for 20 min,transferred (equal weights) into two centrifuge tubes, and centrifugedfor 20 min at 10,000 rpm and 4° C. The clear, blue-green supernatantcontaining unreacted excess PAH/1.00 M NaCl (aq) solution was decantedand discarded, leaving a gray-black pellet of PAH-modified CIGOparticles. Wash water was added to each centrifuge tube in amountsrequired to provide equal tube weights, the tubes were resealed, and theCIGO-PAH pellet was re-dispersed using a vortexer. The samples werere-centrifuged for 20 min at 10,000 rpm (4° C.) and the clear, colorlesswash water was discarded from the pellet to complete the first washcycle. The CIGO-PAH pellet was subjected to 3 such additional washcycles.

After the final wash, weight measurements indicated that ˜45-50 mgCIGO-PAH were lost during processing, leaving ˜75-80 mg CIGO-PAH in thefinal pellets. The pellets were combined, re-suspended in ˜80 mL waterto provide a ˜1 mg CIGO-PAH.mL⁻¹ aqueous stock dispersion, and stored intightly sealed BLUE MAX™ 50 mL Falcon® polypropylene conical tubes untilneeded for film depositions. Unused ˜1 mg CIGO-PAH.mL⁻¹ aqueous stockdispersions were typically discarded after ˜6 days and replaced byfreshly prepared dispersions for film depositions unless notedotherwise.

CIGO-PAH Particle Concentration Determination

Freshly prepared 1 mg CIGO-PAH.mL aqueous dispersion was diluted in aseries with water until concentrations in the optimal 2-4×10⁸particles·mL⁻¹ range observable by the NanoSight LM10-HSBF nanoparticlecharacterization system were obtained. A 60 sec video of particleBrownian motion was taken at room temperature and analyzed to obtain aparticle count using the Nanosight NTA 2.2 software. Brightness and gainwere the same for all samples and the detection threshold wasautomatically adjusted by the software. A blur size of 5 pixel×5 pixelwas chosen for all samples. CIGO-PAH particle concentration in theoriginal sample was then calculated from the dilution factors (˜1/2000)used and the recorded particle counts.

CIGO-PAH Particle Size and Stability Determination

A 3 mL aliquot of ˜1 mg CIGO-PAH.mL⁻¹ aqueous dispersion contained in a1.00 cm pathlength cuvette was used for each dispersion particle sizeand stability determination. Ten DLS measurements were taken andaveraged in any time point at 22° C. Two stability studies wereperformed. For long term stability determination, data was acquired oncea day until day 13 for the dispersion. A second stability study wasconducted for ˜1 mg CIGO-PAH.mL⁻¹ aqueous dispersion containing ˜20 mMTris (pH 8.25) buffer. For this study, 60 μL of 1.00 M Tris pH 8.25aqueous buffer was mixed with 3 mL of the freshly prepared ˜1 mgCIGO-PAH.mL⁻¹ aqueous dispersion. Data was collected continuously for aperiod of 2 h, after which measurements were taken every 30 min for aperiod of 8 hr. All samples were kept at room temperature and ambientpressure during the experiments.

Fabrication of PSS/CIGO-PAH Multilayer Films

Sealable, grooved plastic containers designed for storage of microscopeslides were used as sample containers for the quiescent hand dipcoatedLbL PSS/CIGO-PAH multilayer depositions. Containers were securelypositioned such that substrates were vertically oriented duringdepositions to minimize the chances of binding aggregates orprecipitates formed during treatments. Sufficient quantities of 1 mgCIGO-PAH·mL⁻¹ dispersion and 5 mg PSS·mL⁻¹ 1.00 M NaCl (aq) solution tocover the substrates (i.e., EDA-coated Si wafers and quartz slides) to adepth of ˜25-30 mm were placed in separate containers. EDA-coated Siwafers (i.e., Si-EDA) and EDA-coated quartz slides (i.e., Q-EDA) werethen placed directly into the PSS solution and the container was sealed.After 30 min, the substrates were removed and rinsed for 60 s each withagitation in water. The substrate rinse was repeated two additionaltimes in fresh water before drying in the filtered N₂ gas stream.

Samples were then transferred to container holding the CIGO-PAHdispersion for deposition of the CIGO-PAH nanoparticles. Samples weretypically treated for ˜5-6 h at room temperature. Overnight treatments(i.e., ≧12 h) resulted in no further appreciable particle deposition(i.e., <2-3%), as measured by UV absorbance spectroscopy at 225 nm or285 nm. Following the CIGO-PAH treatment, the samples were washed anddried as described for the PSS treatment. This cycle was repeated tobuild PSS/CIGO-PAH multilayers on the substrate surface.

For the Mo substrate, a polydopamine (PDA) coating was deposited priorto deposition of CIGO-PAH and PSS. Polymerization of dopamine wasinitiated by addition of a 100 μL aliquot of 1.00 M Tris pH 8.25 (aq)buffer to 10 mL of a freshly prepared ˜1 mg dopamine hydrochloride·mL⁻¹(aq) solution. The Mo substrate was treated with this solution for ˜70min, then removed, rinsed thoroughly three times in fresh water, anddried in the filtered N₂ gas stream. The PDA-coated Mo substrate wasthen treated as described above with CIGO-PAH to initiate the multilayerdeposition.

For multilayers fabricated using the robot dipcoater, ˜45-50 mL aliquotsof 1 mg CIGO-PAH·mL⁻¹ dispersion and 5 mg PSS.mL⁻¹ 1.00 M NaCl solutionwere placed into separate treatment beakers. The robot controls were setto refill each of 6 rinse beakers with 70 mL water after each use.Substrates were loaded onto the sample holder and spun during treatmentsand rinses at 140±5 rpm. Substrates were treated with PSS solution for30 min and CIGO-PAH dispersion for 90 min, which was the maximumtreatment time allowable. The substrates were rinsed 60 s in each of 3beakers containing fresh water and dried for 90 s in a stream offiltered N₂ gas following deposition of each PSS or CIGO-PAH layer. TheCIGO-PAH dispersion and PSS solution were replaced and the sample andrinse beakers thoroughly rinsed with water and dried after deposition ofevery 4-6 PSS/CIGO-PAH bilayers. Multilayers fabricated using the robotdipcoater were left standing in air in the closed sample chamberfollowing completion of a deposition cycle until the initiation of thenext cycle.

For hand dipcoated multilayers comprising large numbers of PSS/CIGO-PAHbilayers requiring fabrication times of days or weeks, samples bearingpartially complete films were stored in PSS solution overnight (i.e.,˜12 h). Because PSS depositions were essentially complete (i.e., 100%)within ˜30 min under our conditions, no further PSS deposition occurredovernight. The absorbance spectra of PSS/CIGO-PAH multilayers on Q-EDAsubstrates were periodically recorded as the depositions proceeded tomonitor film growth. CIGO-PAH dispersion and PSS solution werereplenished with fresh aliquots after deposition of every 4-6PSS/CIGO-PAH bilayers. Completed PSS/CIGO-PAH films were stored insealed Fluoroware™ containers until needed for additional experiments.Sample containers holding the CIGO-PAH dispersion and PSS solutions werecleaned after completion of film depositions by soaking overnight in 6 MHCl (aq) solution, which completely dissolved the CIGO-PAH particles,and water, respectively, followed by copious rinsing with water anddrying in the filtered N₂ gas stream prior to re-use.

Fabrication of PDA/CIGO-PAH Multilayer Films

The preparation of the PDA/CIGO-PAH multilayers proceeded in similarfashion to the fabrication of the PSS/CIGO-PAH films with the changesnoted below. First, freshly prepared PDA was formed in situ duringsubstrate treatment by the polymerization of dopamine in basic aqueoussolution for the deposition of each PDA layer. A stock aqueous solutioncontaining 1 mg dopamine HCl.mL⁻¹ was prepared and stored sealed in thedark at 2-4° C. for up to 3 days until needed. A 10 mL aliquot of thestock dopamine HCl solution was allowed to warm to room temperatureduring ˜10-15 min, after which 100 μL of 1.00 M Tris pH 8.25 (aq) bufferwas added with mixing to initiate PDA polymerization. The substrates tobe treated were immediately placed in the solution and allowed to standfor 45 min, during which time the solution developed a brownish color.The substrates were removed from the PDA solution, which was discarded,and rinsed and dried as described for the PSS/CIGO-PAH multilayers. Thecontainer holding the PDA solution was rinsed thoroughly with water anddried before re-use for the next PDA layer deposition using freshlyprepared PDA. The container was cleaned after completion of filmdepositions by immersion in KOH-saturated isopropanol for ˜30 min,followed by copious rinsing with water and drying in the filtered N₂ gasstream prior to re-use.

Second, the 1 mg CIGO-PAH.mL⁻¹ aqueous dispersion was made basic byaddition of 150 μL 1.00 M Tris pH 8.25 buffer to 7.5 ml of the CIGO-PAHdispersion immediately prior to use for film depositions. The PDA-coatedsubstrates were treated 45 min with this CIGO-PAH/Tris pH 8.25dispersion. Each CIGO-PAH/Tris pH 8.25 dispersion aliquot was used forfabrication of 2-3 PDA/CIGO-PAH bilayers before replacement with a freshdispersion aliquot. Lastly, unlike the PSS/CIGO-PAH multilayers,PDA/CIGO-PAH multilayers terminated with a CIGO-PAH layer were storeddry overnight in Fluoroware™ containers until depositions resumed thenext day.

Film Oxidation, Sulfurization, and Photovoltaic Test Device Fabrication

PSS/CIGO-PAH and PDA/CIGO-PAH multilayer films on Si-EDA, Q-EDA, and Mosubstrates were thermally annealed 5 h at 550° C. in air to removeorganic components such as PSS, PAH, and PDA and complete the oxidationof the CIGO particles in the films. The oxidized films were converted tothe corresponding CIGS films via sulfurization by thermal treatment at550° C. in a flowing 20 ccm H₂S gas stream for 3 h. After film oxidationand sulfurization, the resulting CIGS absorber films were characterized.

CIGO Particles

FSP is a non-vacuum gas phase process capable of producing high purityoxide nanoparticles on an industrial scale. Volatile or aerosolizedliquid fuel containing nanoparticle precursor materials is sprayed intoa flame, where the droplets evaporate and undergo combustion. Thespecies formed are rapidly quenched as they leave the reaction zone anddeposit as nanoparticle oxides on a cold collector surface. Gram scalequantities of copper-indium-gallium oxide (CIGO) nanoparticles wereproduced, having an average composition CuIn_(x)Ga_(1-x)O₂, from ethanolsolution containing metal nitrate precursors. Although nanoparticleshaving compositions with 0≦x≦1 can be prepared, experimentation wasfocused on nanoparticles having x≅0.7 (as the absorbance of CIGS filmsof similar composition are well matched to the solar spectrum).

FIG. 1A shows the TEM of the as-prepared CIGO nanoparticles. Looseaggregates comprising ˜10-75 nm diameter particles are typicallyobserved. XRD results shown in FIG. 1B reveal broadened peaks and peakpositions consistent with a mixture of oxides, rather than a singlephase for the as-formed CIGO particles. Thermogravimetry of theas-formed CIGO particles resulted in a weight gain of ˜2% during heatingto 600° C., consistent with the presence of mixed oxides and/orincompletely oxidized metal species within the particles. Nevertheless,sulfurization readily transformed this material into the correspondingCIGS species, as confirmed by its characteristic XRD pattern in FIG. 1B.A literature method for stabilizing the aqueous CIGO particledispersions was adapted for using these particles for film fabrication.(Lvov et al., “Converting Poorly Soluble Materials into Stable AqueousNanocolloids,” Langmuir, 27, 1212-1217 (2011)).

CIGO-PAH Particle Preparation

Amine ligands such as oleylamine are often used as components duringthermal syntheses of various semiconductor and metal nanoparticles.Amines readily bind to metal ion surface sites on the growingnanoparticle, controlling nanoparticle size and providing nanoparticlestability during subsequent particle dispersion in solvents. However,because the nanoparticles were prepared via FSP, incorporation of amineligand during nanoparticle formation was not possible. Therefore,thermally activated amine ligand binding to the as-formed particles wasinvestigated.

Polyamines were selected for further experimentation. Polyamines providethe potential for more rapid surface binding by virtue of their largernumbers of amine sites, with unbound amine sites remaining available forinteraction with water to stabilize the particle dispersion. Moreover,the ability to control surface charge density of the bound polyamine viasolution pH changes provides the requisite cationic surface chargenecessary for particle use in the electrostatic LbL film assemblyprocess, while also further enhancing particle dispersion stability.Also, the ability to control thickness of the bound polyamine viachanges in polymer molecular weight and solution ionic strength providesa deformable coating expected to facilitate particle packing during filmdeposition.

Initial work focused on polyethylenimine (PEI), a high molecular weight(750,000 g·mole⁻¹) branched polyamine, as the CIGO particle coating.Lvov et al. [43] have shown that sonication of colloids in the presenceof polyelectrolytes provides the necessary thermal activation forefficient attachment of polyelectrolyte to the colloid particle surface.(Lvov et al., “Converting Poorly Soluble Materials into Stable AqueousNanocolloids,” Langmuir, 27, 1212-1217 (2011)). In addition, sonicationagitates and suspends the particles during treatment to promotedeposition of a more uniform polyelectrolyte coating.

A sonication treatment was carried out using PEI in 1.00 M NaCl (pH˜8.2) solution to promote deposition of thicker films onto the particlesurface. Thicker films are deposited from solutions of higher ionicstrength because increased counterion pairing with protonated aminesites at higher salt concentrations partially neutralizespolyelectrolyte charge. As repulsive electrostatic interactions amongpolymer protonated amine sites decrease, van der Waals and relatedforces become more important leading to polymer chain coiling. Bindingof such coiled chains to the particle surface creates a thicker polymercoating on the nanoparticle due to steric repulsions between chains ofadjacent polymers, providing additional unbound amine sites forinteraction with solvent as required to stabilize particle dispersions.In contrast, extended chains occurring at low ionic strength due toelectrostatic repulsions among adjacent protonated amine sites in thepolymer produce thinner films with fewer unbound amine sites capable ofproviding stabilizing interaction with the solvent.

Various experiments under different sonication conditions indicated that˜30 min sonication treatment of CIGO particles at ˜480 W power inaqueous PEI/1.00 M NaCl (pH ˜8.2) solution was sufficient to bind PEI tothe CIGO surface, as evidenced by formation of a stable CIGO-PEIdispersion. Although the temperature of the particle dispersion aftersonication was typically 50-55° C., sonication can produce localtemperatures ranging from 100s-1000s of degrees which are more thansufficient to promote the amine binding reaction. Cooled dispersionsexhibited decreases in liquid volume of ˜20-25% consistent with heatingand appeared gray in color.

During subsequent centrifugation to remove and rinse excess PEI/1.00 MNaCl (pH ˜8.2) solution from the CIGO-PEI particles, there was a highdegree of particle aggregation indicated by difficulty in re-dispersingthe pellet in the wash water. This behavior was attributed to (1)potential non-covalent interactions, such as amine-amine hydrogenbonding and chain entanglements, between extended PEI polymer chains onadjacent particles, and (2) covalent bridging of adjacent nanoparticlesby a single surface bound PEI chain. Replacement of branched PEI withlinear polyallylamine (PAH) of lower molecular weight (i.e., ≧120,000g·mole⁻¹) reduced particle aggregation during processing, and freezedrying of isolated CIGO-PAH particles further inhibited re-dispersion.Subsequent experiments with PAH of various molecular weights eventuallyshowed that 8,000-11,000 g·mole⁻¹ material provided particles readilydispersible during processing that also remained sufficiently stable asaqueous dispersions for subsequent work. Amine binding to the CIGOparticle surface was confirmed by a control experiment in which PSSreplaced PAH during sonication. CIGO particles treated with PSS rapidlysettled following sonication, consistent with the negligible metalligating ability of the PSS sulfonate group compared to the PAH aminegroup.

CIGO-PAH Particle Dispersion Characterization

The CIGO-PAH particles were characterized by several techniques toassess their stability and suitability for LbL deposition. The presenceof a PAH coating on the CIGO particles is inferred by their formation ofstable aqueous dispersion and electrostatic binding of the particles toa PSS film deposited onto an EDA-coated quartz slide. In contrast to the˜2% weight gain observed for bare CIGO particles, thermogravimetryindicates a ˜6% weight loss for CIGO-PAH particles consistent withoxidation and loss of the organic coating during a temperature ramp to550° C.

Measurements of Brownian motion of CIGO-PAH particles dispersed inwater, made using a NanoSight LM10-HSBF nanoparticle tracking system,indicated that a 1 mg CIGO-PAH. mL⁻¹ dispersion contained ˜(9±2)×10 ¹¹particles·mL⁻¹ (n=8), with an average particle size of ˜125±15 nm andparticle distribution shown in FIG. 2A. While light scattering resultsmeasured the total particle diameter comprising the CIGO particle andPAH coating, the ˜125±15 nm particle size noted for the CIGO-PAHparticles compared to the as-prepared ˜10-75 nm diameter CIGO particles(FIG. 1A) suggests that some PAH-assisted particle aggregation occurredduring sonication despite the use of the lower molecular weight PAHspecies. The UV-visible absorbance spectrum corresponding to theCIGO-PAH dispersion, shown in FIG. 2B was characterized by the rapidlyrising absorbance with decreasing wavelengths expected for particlelight scattering behavior. The spectrum also features a distinct peak at285 nm, which is not present in the thermally annealed CIGO particles.Because PAH exhibits no absorbance at wavelengths ≧200 nm, this peak wasassigned to incompletely oxidized species consistent with the weightgain observations during thermogravimetric oxidation of the as-preparedCIGO samples.

Stability of the CIGO-PAH dispersion was also assessed as apre-requisite for its use in the fabrication of PSS/CIGO-PAH andPDA/CIGO-PAH multilayer films. For the CIGO-PAH dispersions used in thepreparation of PSS/CIGO-PAH films, changes in particle size distributionover a 13 day period were monitored by dynamic light scattering (DLS).The appearance of particles larger than 300 nm, which were not observedin the freshly prepared dispersion, was set as the aggregationthreshold. Aggregates were detected in the dispersion prior to mixingonly after 3 days, as shown in FIG. 3A. However, aggregates were nolonger detected upon mixing the sample, indicating that aggregation atthis stage was still reversible (FIG. 3B). In contrast, after 6 days noaggregates were detected in the quiescent sample until the sample wasmixed. Aggregates were then detected 10 min after mixing, as shown inFIG. 3C and persisted for at least 40 min (FIG. 3D) indicative ofirreversible aggregation. Even larger aggregates were detected at days 7and 13 after mixing followed by a 10 min wait, consistent with furtherdestabilization of the dispersion as shown in FIGS. 3E and 3F. DLSmeasurements indicated that <5% of the original particles separated fromthe dispersion as a result of settling during the course of theexperiment, a value that did not materially alter particle bindingkinetics under the deposition conditions.

The stability of freshly prepared aqueous 1 mg CIGO-PAH.mL⁻¹ 20 mM TrispH 8.25 buffered dispersion used for the deposition of the PDA/CIGO-PAHfilms was also evaluated, under quiescent conditions, by DLSmeasurements during an 8 h aging experiment. The average size of themost abundant particles in the dispersion during 8 h was 137±22 nm (n=22particles). Some particles larger than 300 nm were first detected by DLSafter 30 min and after 50 min some micron size particles were observed,as shown in FIG. 4. Aggregation occurred in periodic fashion in whichmicron size particle levels initially increased to a threshold value,then dropped precipitously as the large aggregates settled out of thedispersion before the cycle began anew. Quantities of micron sizedaggregates were detected after 50, 110, 220, and 420 min of aging, withfew if any noted at intermediate times consistent with this mechanism.The growth and presence of micron size particles confirmed that Tris pH8.25 buffer promotes particle aggregation as the dispersion ages,behavior consistent with partial PAH deprotonation (pK_(a)˜9.5-10) at pH8.25 leading to loss of stabilizing positive charge sites on theCIGO-PAH particle. In contrast to the CIGO-PAH dispersion study, asomewhat larger fraction of particles were removed from theCIGO-PAH/Tris pH 8.25 dispersion (<10%; Table S1) via aggregation andsettling. Nevertheless, the CIGO-PAH/Tris pH 8.25 dispersion remainedsufficiently stable for use in film fabrication under the depositionconditions.

PSS/CIGO-PAH and PDA/CIGO-PAH Multilayer Depositions

FIG. 5 illustrates the general approach for fabrication, as well asoxidation and sulfurization, of the PSS/CIGO-PAH and PDA/CIGO-PAHmultilayers. For reproducible fabrication of PSS/CIGO-PAH multilayers,appropriate substrate treatment times for reproducible deposition ofCIGO-PAH and PSS layers were determined. For this purpose, aQ-EDA/(PSS/PAH)₃/PSS film was prepared using the 5 mg PAH.mL⁻¹ 1.00 MNaCl (aq) and 5 mg PSS.mL⁻¹ 1.00 M NaCl (aq) solutions as described inDressick, et. al., “Divalent-Anion Salt Effects in PolyelectrolyteMultilayer Depositions”, Langmuir, 28, 15831-15843 (2012). Treatment ofthis base film with the CIGO-PAH dispersion indicated that CIGO-PAHparticle deposition, as measured by 285 nm film absorbance, was >96%complete after 4 h and >98% complete after 6 h compared to a filmdeposited overnight (i.e., ≧12 h). Similar adsorption studies involvingtreatment of the CIGO-PAH layer with the 5 mg PSS.mL⁻¹ 1.00 M NaCl (aq)solution indicated that PSS binding was complete (i.e., 100%) within 30min. Consequently, substrate treatment times of 30 min and ≧5 h wereselected for hand dipcoating depositions of the PSS and CIGO-PAH layers,respectively.

Once treatment times had been established, PSS/CIGO-PAH multilayerdepositions were continued using the PSS/PAH base film to confirm layerdeposition reproducibility and assess the effects of CIGO-PAH dispersionage on film growth. Film fabrication was initiated under quiescent handdipcoating conditions using a freshly prepared stock CIGO-PAHdispersion, which aged 9 days during the time required to complete thefilm. The CIGO-PAH treatment dispersion was replaced by decantation withthe aging CIGO-PAH stock dispersion after every 3 days of use. Care wastaken not to transfer any precipitated material present into thetreatment container, which was cleaned before the dispersion wasreplaced. Fabrication of a second film was also separately initiatedafter the CIGO-PAH stock dispersion was aged 2 days, with the finallayers deposited using a CIGO-PAH dispersion aged 11 days.

FIG. 6A shows the absorbance spectrum of a completed 18 bilayerPSS/CIGO-PAH multilayer of structure Q-EDA/(PSS/PAH)₃/(PSS/CIGO-PAH)₁₈,together with spectra of intermediate films having 6 and 12 bilayers.Films having these structures are present on each side of the quartzsubstrate. The spectra are similar to that of the aqueous CIGO-PAHdispersion shown in FIG. 2A with the exception of an additional peaknear 225 nm characteristic of the PSS layers. The absorbance spectrum ofthe 18 bilayer film after thermal annealing is also shown. The UVintensity is decreased and the PSS absorbance at 225 nm is absent afterannealing due to the removal of the organic PAH and PSS components. Inaddition, the 285 nm peak, assigned to incompletely oxidized specieswithin the CIGO particles, is also absent, consistent with thethermogravimetry results.

FIG. 6B shows the changes in PSS/CIGO-PAH film absorbance, monitored at285 nm where CIGO is the predominant absorbing species and at 225 nmwhere both CIGO and PSS absorb well, as a function of the number ofbilayers deposited for both films. In each case, absorbance increasesnonlinearly with slight upwards curvature as the number of bilayersdeposited increases. The amount of material deposited is slightly butconsistently larger for the film initiated using the 2-day old CIGO-PAHthan the fresh CIGO-PAH dispersion. In addition, traces of settledCIGO-PAH precipitate are observed at the bottom of the CIGO-PAHcontainers following completion of the films, behaviors in agreementwith the aggregation noted for the aqueous CIGO-PAH dispersion in theDLS studies. Results suggest the use of CIGO-PAH dispersions aged atmost 6 days, and preferably 3 days or less, optimize linearity andreproducibility during PSS/CIGO-PAH film depositions again consistentwith the DLS results.

Experiments were also performed using a robot dipcoater to demonstratethe ability to automate film fabrication directly onto substrates foruse in a manufacturing environment. Because software constraintsassociated with the robot limited deposition times to no more than 90min per layer, substrate treatments were carried out with stirring toenhance deposition rates. A 90 min CIGO-PAH deposition with samplestirring (˜140 rpm) was found to correspond approximately to a 6 hquiescent CIGO-PAH treatment, as measured by the 285 nm absorbance ofthe deposited CIGO-PAH material. However, initial attempts to directlydeposit multilayers bearing more than 8 PSS/CIGO-PAH bilayers led tocomplete settling of the CIGO-PAH dispersion. The problem was eventuallytraced to cumulative carryover of small amounts PSS, identified by itscharacteristic UV spectrum and 225 nm absorbance band into the CIGO-PAHdispersion. This apparently occurred due to incomplete draining of rinsewater during rinse water refill cycles.

Acceptable films could nevertheless be deposited if the CIGO-PAHdispersion and PSS solution were replaced after deposition of every 4-6bilayers, provided that the sample and rinse beakers were adequatelyrinsed and dried before re-use. If the used CIGO-PAH dispersion wasreplaced with dispersion aged 3 days or less, in which aggregate levelsare negligible as measured by DLS, further improvements in film qualityand reproducibility accrued. Specifically, nearly linear increases infilm absorbance as a function of the number of bilayers deposited werenow obtained for films comprising as many as 80 bilayers (on each sideof the quartz slide), as shown in FIG. 7A. Subsequent oxidation andsulfurization of the film provided an absorber layer capable of completeabsorption of visible and near IR light (i.e., absorbance>2 for allwavelengths≦1100 nm), as shown in FIG. 7B.

Although PSS/CIGO-PAH films were successfully deposited, coatingmigration onto areas of Mo not originally covered by the film was notedfollowing sulfurization. This behavior was not observed for acorresponding film deposited on a quartz substrate. It suggested thatadhesion of the PSS/CIGO-PAH multilayer to the underlying Mo substratemay be insufficient to limit film migration. In fact, an XPS studyindicated that neither organothiol and organosiloxane monolayers norpolyamines were readily chemisorbed to the Mo surface (though PSS wasweakly bound), behavior consistent with this hypothesis.

Therefore, a polymeric replacement for PSS was sought that could morestrongly bind CIGO-PAH particles to each other and the substrate inthese multilayer films. PDA is a strong adhesive produced bypolymerization of dopamine under basic pH conditions and used by marineorganisms such as mussels to anchor themselves to solid surfaces inunderwater environments. Although its composition and structure remainuncertain, electrostatic and covalent interactions involving both theamine and catechol sites of dopamine have been identified within PDA.Therefore, it was expected that amine sites on the CIGO-PAH particleswould be effectively bound in films containing a PDA component.

Initial experiments revealed deposition of a yellow-brown PDA film ontoQ-EDA, Si-EDA, and even Mo substrates treated for extended periods withan aqueous 1 mg dopamine.mL⁻¹ solution containing 10 mM Tris pH 8.25buffer, as shown in FIG. 8A. A nearly linear deposition rate during thefirst ˜45 min was observed, as shown in FIG. 8B, as PDA polymerizationoccurred on both sides of the EDA-coated quartz substrate. Thereafter,deposition continued at a slower rate (˜50-60%) suggesting that coverageof the EDA was complete and further reaction occurred on the alreadydeposited PDA layer. However, attempts to bind the CIGO-PAH dispersiondirectly onto PDA films deposited for 45 min or longer led toirreproducible results. CIGO-PAH was reproducibly deposited only whenthe dispersion contained ˜20 mM Tris pH 8.25 buffer, indicating that thepresence of catecholate anions on the PDA surface was a pre-requisitefor CIGO-PAH binding. Subsequent time dependent studies indicated thatCIGO-PAH particle deposition onto a PDA film was essentially complete inas little as 45 min, a substantially shorter time than the ˜5 htreatments required for deposition in quiescent hand dipcoatingexperiments.

Fabrication of PDA/CIGO-PAH films directly on the substrates viaquiescent hand dipcoating was successfully performed using 45 mintreatments for both the PDA/Tris pH 8.25 solution and the CIGO-PAH/20 mMTris pH 8.25 dispersion. By freshly preparing 1 mg dopamine.mL⁻¹ 10 mMTris pH 8.25 solution for deposition of each PDA layer and using a newaliquot of CIGO-PAH/20 mM Tris pH 8.25 dispersion, prepared by additionof 1.00 M Tris pH 8.25 buffer to 1 mg CIGO-PAH.mL⁻¹ dispersion aged ≦3days, after deposition of every 3-4 PDA/CIGO-PAH bilayers, a nearlylinear and reproducible multilayer deposition was achieved per FIG. 9A.An absorbance spectrum for the PDA/CIGO-PAH film of structureQ-EDA/(PDA/CIGO-PAH)₂₀ present on each side of the quartz slide is shownin FIG. 9B. Spectra are also shown in FIG. 9B for the correspondingoxidized and sulfurized films, which are similar to the analogous filmsprepared from PSS/CIGO-PAH multilayers in FIG. 7B.

Somewhat thicker films of structure Si-EDA/(PDA/CIGO-PAH)₂₆/PDA werealso deposited via hand dipcoating on Si-EDA wafers for characterizationof film morphology and topography. Top-view and side-view SEM images ofthe Si-EDA/(PDA/CIGO-PAH)₂₆/PDA film as deposited and after oxidationand sulfurization are shown in FIG. 10. The as-deposited film in FIG.10A reveals a sponge-like morphology comprising aggregates of ˜100-200nm diameter CIGO-PAH particles cemented together by PDA. The side-viewof the film shown in FIG. 10B indicates that nanochannels havingdiameters comparable to the CIGO-PAH particle sizes are present andcompletely penetrate the film, consistent with inefficient packing ofthe particles during the deposition process. Film thickness is ˜1500 nm,with a roughness of ±150 nm again consistent with the presence of porestructures and inefficient particle packing. Although film thickness androughness (i.e., ˜1300±100 nm) are each reduced somewhat following airoxidation to remove the PDA and PAH components (FIGS. 10C and 10D),porosity, particle size, and general film morphology are little changed.

However, significant changes were observed following sulfurization.Sulfurization of the homogeneous porous CIGO film of FIGS. 10C and Dunexpectedly altered the morphology of the resulting CIGS film. Insteadof an expected compact, flat CIGS film the result was a unique thinnedCIGS film with microscale tall CIGS spikes/fibrils. Lighter regions ofcoalesced material were clearly observed, together with ˜100 nmparticles and pores, on the top surface of the film followingsulfurization in FIG. 10E. The side-view of the same film in FIG. 10Fshows that the lighter material comprises coiled and rod-shaped fibrilsseveral microns in height distributed over the top surface of the film.A close-up of the film side-view in FIG. 10G indicates that the filmthinned significantly, with film thickness in regions absent the fibrilsnow just ˜600±100 nm and nanochannels penetrating the film to theunderlying Si substrate clearly seen. These observations arecollectively consistent with a redistribution of material during thesulfurization process, as also noted during sulfurization of thePSS/CIGO-PAH films deposited on Mo substrates. The phenomenon is clearlyassociated with the presence of the substrate and/or polyelectrolytes,since as-prepared CIGO particles are cleanly sulfurized to CIGS (FIG.1B) without dramatic morphology changes.

The composition of the sulfurized film shown in FIGS. 10E-10G isverified by the XRD analysis in FIG. 11, which exhibits the reflectionpeaks expected for CIGS material. No evidence for additional phases ormaterials is observed, indicating that the fibrils and top surfaceparticulates comprise the same material. This is further confirmed byEDS analyses performed on both the particulate and fibrillar regions.The former exhibits an elemental composition (in atom %) of Cu=18.35%,In=21.05%, Ga=5.07%, and S=45.26%. A 10.27% oxygen value is alsoobserved, consistent with sampling of the underlying substrate oxidethrough the film nanochannels during the measurement. EDS of thefibrillar regions provides a substantially identical composition ofCu=18.73%, In=20.87%, Ga=5.50%, and S=47.10% (with O=7.81%). Compared tothe target CIGS composition of Cu=25%, In=17.5%, Ga=7.5%, and S=50%expected from the CIGO precursor of composition CuIn_(0.7)Ga_(0.3)O₂,this CIGS material is indium rich but both Cu and Ga poor. This resultis consistent with a previous observation of a blue-green PAHsupernatant suggestive of mixed chloro- and amine-complexes of Cu(II)following CIGO particle sonication in PAH, indicating that theas-prepared CIGO particle composition may need to be adjusted during FSPto account for selective metal ion extraction during polyelectrolytebinding.

Alternatives

Not wishing to be bound by theory, there are a variety of possiblealternatives. For example, CIGX′ (X′═S, Se or Te) particles may be usedin place of CIGO nanoparticles, in which case a sulfurization (orselenizationor tellurization) step at the end of the process may beomitted. Polyamines other than PAH can be used, provided that theypossess a group reactive with at least one of the Cu, In, and/or Gasurface sites of the CIGX (X═O, S, Se, or Te) nanoparticles and aresufficiently hydrophilic and charged (1) to stabilize the resultingaqueous CIGX-polymer dispersion and (2) electrostatically bind to theoppositely-charged substrate during film fabrication. For example,polyethylenimine (PEI) and polylysine (PL) have alkylamine groupscapable of binding the particles and are useful. Other hydrophilicpolypeptides or proteins having a plurality of like-charged side chainsand at least one accessible terminal amino group represent otherpossibilities. A polycarboxylate, such as polyacrylate, which binds theCu, In, or Ga sites on the CIGX nanoparticle surface less strongly isless preferred. Hydrophilic copolymers in which at least one polymercomponent has such reactive groups would also be useful. For example,PSAX, a polyvinyl alcohol having a fraction, f (f<<1) of its R—OH groupsconverted to xanthates (i.e., R—O—CS₂) capable of covalently binding theCIGX surface with its remaining R—OH groups converted into chargedspecies by reaction, e.g., with succinic anhydride to maintain ahydrophilic anionic coating environment would be acceptable. Inaddition, in situ polymerization of dopamine in aqueous solution in thepresence of CIGX particles to coat said particles with an anionicpolydopamine (PDA) coating would provide polyanionic CIGX particlesuseful for film fabrication, provided that polymerization yields acoating having sufficient numbers of charged catecholate anion groups toform a stable dispersion. Films could also be prepared using polyaminecations such as those mentioned above, as well as non-coordinatingpolyamines such as polydiallyldimethyalammonium chloride (PDDA), as thepolycationic component together with a CIGX particle dispersion in whichthe CIGX nanoparticle is coated by a covalently bound polyanion such asPSAX or PDA. This provides an alternate way to build a film using apolycation solution and polyanion-coated CIGX nanoparticle dispersion.In addition, films could be built using both polycationic andpolyanionic CIGX dispersions, such as CIGX-PAH as the cationic componentand CIGX-PSAX or CIGX-PDA as the anionic component. Films might also bebuilt using charged organothiols or dithiocarbamates chemisorbed to theCIGX nanoparticles, provided that the particles are sufficiently smallthat the charged coating can maintain a stable aqueous dispersion.Alternatives also are possible for the layer-by-layer depositionprocedure, including spray coating, spincoating; hand dipcoating, and/orrobot dipcoating of solutions and suspensions as is known to thosefamiliar with the state of the art. Finally, it should be possible toremove the polyelectrolyte film component and sulfurize a filmcontaining CIGO nanoparticles simultaneously by sulfurizing under aninert atmosphere at high temperature.

The above descriptions are those of the preferred embodiments of theinvention. Various modifications and variations are possible in light ofthe above teachings without departing from the spirit and broaderaspects of the invention. It is therefore to be understood that theclaimed invention may be practiced otherwise than as specificallydescribed. Any references to claim elements in the singular, forexample, using the articles “a,” “an,” “the,” or “said,” is not to beconstrued as limiting the element to the singular.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method of making copper indium galliumchalcogenide thin films, comprising: producing copper-indium-galliumoxide (CIGO) nanoparticles via flame spray pyrolysis; binding the CIGOnanoparticles to a polyamine and dispersing the polyamine-CIGOnanoparticles in an aqueous solution to form a polyamine-CIGOdispersion; making a polyanion solution; and dipping a substrate intothe polyanion solution and then the polyamine-CIGO dispersion, whereinalternate dipping between the polyanion solution and the coated CIGOdispersion may be repeated multiple times to form a CIGO film.
 2. Themethod of claim 1, additionally comprising oxidizing thepolyamine-CIGO/polyanion film.
 3. The method of claim 1, additionallycomprising sulfurizing the polyamine-CIGO/polyanion film to convert saidfilm to a copper indium gallium sulfide film.
 4. The method of claim 1,additionally comprising selenizing the polyamine-CIGO/polyanion film toconvert said film to a copper indium gallium selenide film.
 5. Themethod of claim 1, additionally comprising tellurizing thepolyamine-CIGO/polyanion film to convert said film to a copper indiumgallium telluride film.
 6. The method of claim 1, wherein the polyaminecomprises polyallylamine (PAH).
 7. The method of claim 1, wherein thepolyanion comprises polystyrenesulfonate (PSS) or polydopamine (PDA). 8.The method of claim 1, wherein the substrate comprises silicon, quartz,or molybdenum.
 9. The method of claim 8, wherein the silicon or quartzsubstrate has a coating comprisingN-(2-aminoethyl)-3-aminopropyltrimethoxysilane (EDA) and the molybdenumsubstrate remains uncoated or has a coating of polydopamine (PDA).
 10. Acopper indium gallium chalcogenide thin film made by the methodcomprising: producing copper-indium-gallium oxide (CIGO) nanoparticlesvia flame spray pyrolysis; binding the CIGO nanoparticles to a polyamineand dispersing the polyamine-CIGO nanoparticles in an aqueous solutionto form a polyamine-CIGO dispersion; making a polyanion solution; anddipping a substrate into the polyanion solution and then thepolyamine-CIGO dispersion, wherein alternate dipping between thepolyanion solution and the coated CIGO dispersion may be repeatedmultiple times to form a CIGO film.
 11. The copper indium galliumchalcogenide thin film of claim 10, additionally comprising oxidizingthe CIGO film.
 12. The copper indium gallium chalcogenide thin film ofclaim 10, additionally comprising sulfurizing the film to convert theCIGO film to a copper indium gallium sulfide film.
 13. The copper indiumgallium chalcogenide thin film of claim 10, additionally comprisingselenizing the film to convert the CIGO film to a copper indium galliumselenide film.
 14. The copper indium gallium chalcogenide thin film ofclaim 10, additionally comprising tellurizing the film to convert theCIGO film to a copper indium gallium telluride film.
 15. The copperindium gallium chalcogenide thin film of claim 10, wherein the polyaminecomprises polyallylamine (PAH).
 16. The copper indium galliumchalcogenide thin film of claim 10, wherein the polyanion comprisespolystyrenesulfonate (PSS) or polydopamine (PDA).
 17. The copper indiumgallium chalcogenide thin film of claim 10, wherein the substratecomprises silicon, quartz, or molybdenum.
 18. The copper indium galliumchalcogenide thin film of claim 17, wherein the silicon or quartzsubstrate has a coating comprisingN-(2-aminoethyl)-3-aminopropyltrimethoxysilane (EDA) and the molybdenumsubstrate remains uncoated or has a coating of polydopamine (PDA).
 19. Aphotovoltaic device comprising a copper indium gallium sulfide thin filmmade by the method comprising: producing copper-indium-gallium oxide(CIGO) nanoparticles via flame spray pyrolysis; binding the CIGOnanoparticles to a polyamine and dispersing the polyamine-CIGOnanoparticles in an aqueous solution to form a polyamine-CIGOdispersion; making a polyanion solution; dipping a substrate into thepolyanion solution and then the polyamine-CIGO dispersion, whereinalternate dipping between the polyanion solution and the coated CIGOdispersion may be repeated multiple times to form a CIGO film; oxidizingthe CIGO film; and sulfurizing the film to convert the CIGO film to acopper indium gallium sulfide film.
 20. The photovoltaic device of claim19, wherein the polyamine comprises polyallylamine (PAH).
 21. Thephotovoltaic device of claim 19, wherein the polyanion comprisespolystyrenesulfonate (PSS) or polydopamine (PDA).
 22. The photovoltaicdevice of claim 19, wherein the substrate comprises silicon, quartz, ormolybdenum.
 23. The photovoltaic device of claim 22, wherein the siliconor quartz substrate has a coating comprisingN-(2-aminoethyl)-3-aminopropyltrimethoxysilane (EDA) and the molybdenumsubstrate remains uncoated or has a coating of polydopamine (PDA).
 24. Acomposition of matter, comprising: a silicon substrate coated withN-(2-aminoethyl)-3-aminopropyltrimethoxysilane (EDA); multiplealternating layers of polydopamine (PDA) and polyallylamine boundcopper-indium-gallium oxide nanoparticles on the substrate.
 25. Acomposition of matter, comprising: copper-indium-gallium oxide (CIGO)nanoparticles prepared via flame spray pyrolysis; and polyallylamine(PAH) bound to the CIGO nanoparticles and dispersing the PAH-CIGOnanoparticles to form PAH-coated CIGO nanoparticles.
 26. A copper indiumgallium sulfide (CIGS) thin film made by the method comprising: coatinga silicon substrate with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane(EDA); producing copper-indium-gallium oxide (CIGO) nanoparticles viaflame spray pyrolysis; binding the CIGO nanoparticles to polyallylamine(PAH) and dispersing the PAH-CIGO nanoparticles in an aqueous solutionto form a PAH-CIGO dispersion; making a polydopamine (PDA) solution; anddipping the substrate into the PDA solution and then the PAH-CIGOdispersion, wherein alternate dipping between the PDA solution and thePAH-CIGO dispersion may be repeated multiple times to form a CIGO film;oxidizing the CIGO film; and sulfurizing the film to convert the CIGOfilm to a CIGS film; wherein the CIGS film comprises fibrils on itssurface.